1. Field of the Invention
The present invention relates to a polyamide resin composition, more particularly, to a polyamide resin composition which is suitable as packaging materials for foods, beverages, medicines, electric parts, etc. because of its excellent clarity, appearance, and gas barrier properties.
2. Description of the Prior Art
Polyamide resin has been widely used as the material for injection-molding automotive parts, electric parts, electronic parts, etc. because of its excellent mechanical properties. Its application field extends to packaging materials for foods, beverages, medicines, electric parts, etc. Of various polyamide resins, a m-xylylene group-containing polyamide prepared by the polycondensation of m-xylylenediamine and an aliphatic dicarboxylic acid, particularly, polyamide MXD6 prepared from m-xylylenediamine and adipic acid is utilized as a gas-barrier material for manufacturing a shaped article such as film and bottle because of its low permeability against gaseous substance such as oxygen and carbon dioxide. Recently, there is an increasing demand for a gas-barrier package capable of storing foods, beverages, etc. for a long term without detracting their freshness. This in turn increases the demand for improvement of the gas-barrier properties much more.
As a method for improving the gas-barrier properties of polyamide resin, Japanese Patent Application Laid-Open No. 2-69562 discloses to incorporate a layered silicate into a polyamide resin. In the proposed method, the layered silicate can be uniformly dispersed, for example, in nylon 6 resin to improve the mechanical properties and the gas-barrier properties. This method, however, requires a particular production method and apparatus and can be applied to a limited resin such as a polyamide resin derived from a lactam monomer. Therefore, the method cannot be applied to polyamide MXD6 obtained by the polycondensation of a xylylenediamine monomer and an aliphatic dicarboxylic acid monomer.
Japanese Patent Application Laid-Open Nos. 2-305828 and 8-53572 disclose to melt-knead a polyamide resin and the layered silicate in an extruder. The proposed methods can disperse the layered silicate into the polyamide resin to some extent to improve the gas-barrier properties of the resulting shaped article such as film. However, the dispersibility of the layered silicate becomes insufficient in some cases to make the improvement in the gas-barrier properties insufficient. In addition, the methods likely to form aggregates and gel-like particles and the resulting shaped article is poor in clarity due to its high haze, thereby deteriorating the productivity and the product value.